1. Field of the Invention
The present invention relates to a system and a method of monitoring a condition of an optical fiber transmission line in a wavelength division multiplexing light transmitting system for multiplexing and transmitting signal lights having different wavelengths.
2. Description of the Related Art
In a light transmitting system, a signal light may be attenuated or cut off, in an optical fiber transmission line and in a light amplifying repeater for various reasons. It is, therefore, necessary to monitor a condition of the light transmission line in order to operate the system in an effective manner.
Japanese Laid-Open Patent Application No. 5-344067 describes the following light transmission line monitoring method.
In an up optical fiber transmission line (to be referred to as an, "up line" hereinafter), a signal light having a wavelength of .lambda.1 is output from a transmitter for the up line and transmitted to an receiver for the up line. In a receiver for the up line, the signal light is amplified by a light amplifier, and received by a light receiving section through a band pass optical filter.
A monitor signal on the optical fiber transmission line is superimposed on the signal light by modulation in the transmitter for the up line. The signal light with the monitor signal is introduced into a down optical fiber transmission line (to be referred to as "down line" hereinafter) through a light returning circuit comprising a light attenuator, and is transmitted to a receiver for the down line.
The monitor signals arrive at different times at the receiver for the down line for each returning circuit. Thus, they can be separated by delaying the modulated signal and evaluating a correlation with the monitor signals arriving at the receiver. Accordingly, it is possible to convert a detected monitor signal and to evaluate a correlation with a predetermined standard signal to thereby monitor a loss on the light transmission line between respective repeater intervals.
A wavelength division multiplexing light transmission method is similarly carried out. In this case, the monitor signal is superimposed on at least one multiplexed signal light by modulation.
In the light transmitting system, the monitor signal is superimposed on the signal light by modulation. Thus, when carrying out the monitoring operation at a time when the system is in service, the modulating ratio must be reduced to a low level in order to prevent deterioration of the main signal. On the other hand, a signal-noise-ratio (SNR) of the monitor signal is proportional to a square of the modulation ratio. In a case of a low modulation ratio, this results in a problem, because the SNR of the monitor signal is not sufficient.
When the signal light on which the monitor signal is superimposed by modulation is introduced into the other light transmission line through the light returning circuit, the signal light is sufficiently attenuated in order to prevent the deterioration of the signal light on the light transmission line. In this case, a long time is needed to perform a correlation detection in the receiver in order to detect the attenuated monitor signal. This results in a problem that a trouble point on the light transmission line cannot be promptly specified.
Furthermore, the receiver for the down line, which receives the signal light on which the monitor signal is superimposed, simultaneously receives the signal light transmitted from a transmitter for the down line. Thus, if both transmitters for the up line and for the down line transmit the monitor signals at the same time, both receivers for the up line and for the down line receive the signal lights on which the monitor signals transmitted from the respective transmitters are superimposed. Therefore, monitoring the optical fiber transmission line becomes impossible. For this reason, in the conventional light transmission line monitoring method, both the transmitting and receiving stations having a transmitter and a receiver must alternatively carry out the monitoring operation, respectively. Since it takes a long time to detect the monitor signal, for example, there may be a possibility that each station may only carry out only one monitoring operation per day.